Instructions
Speak to the King and Queen of Feymarch and have them open the warp tile on the upper level of the castle to reach the Passage of the Eidolons. Do this only after you're done in the town of Feymarch and you've collected all the items you can find. The Passage of the Eidolons will take you to the next part of the scenario, so if you've left something undone in Feymarch, you will have to double back.
Navigate through the Passage of the Eidolons until you reach the Dwarven Castle. The Passage is a maze filled with random enemy encounters, so come prepared for a fight. The exit of the Passage that leads to the Dwarven Castle is on the third floor, so fight your way up two levels. When you get outside, you will be attacked by a Zu that can't be killed. Keep fighting it until a cutscene begins.
Enter the Dwarven Castle and go to the third floor, where there is an invisible wall hiding Hermes' Sandals. When you first enter the castle, you will need to speak with King Giott. Once you have, take the west stairs up two levels. On this floor, there is a fake wall. Walk along the edge of the room until you enter a secret chamber. In this chamber are four treasure chests, one of which houses a pair of Hermes' Sandals.